The subject matter disclosed herein relates generally to tuning a motor drive and, more specifically, to a method to isolate the effects of mechanical loading to simplify initial tuning and improve performance in a motor drive system.
As is known to those skilled in the art, motor drives are utilized to control operation of a motor. According to one common configuration, a motor drive includes a DC bus having a DC voltage of suitable magnitude from which an AC voltage may be generated and provided to the motor. The DC voltage may be provided as an input to the motor drive or, alternately, the motor drive may include a rectifier section which converts an AC voltage input to the DC voltage present on the DC bus. The motor drive includes power electronic switching devices, such as insulated gate bipolar transistors (IGBTs), thyristors, or silicon controlled rectifiers (SCRs). The power electronic switching device further includes a reverse conduction power electronic device, such as a free-wheeling diode, connected in parallel across the power electronic switching device. The reverse conduction power electronic device is configured to conduct during time intervals in which the power electronic switching device is not conducting. A controller in the motor drive generates switching signals to selectively turn on or off each switching device to generate a desired DC voltage on the DC bus or a desired motor voltage.
The motor drive receives a command signal which indicates the desired operation of the motor. The command signal may be a desired position, speed, or torque at which the motor is to operate. The position, speed, and torque of the motor are controlled by varying the amplitude and frequency of the AC voltage applied to the stator. The motor is connected to the output terminals of the motor drive, and the controller generates the switching signals to rapidly switch the switching devices on and off at a predetermined switching frequency and, thereby, alternately connects or disconnects the DC bus to the output terminals and, in turn, to the motor. By varying the duration during each switching period for which the output terminal of the motor drive is connected to the DC voltage, the magnitude of the output voltage is varied. The motor controller utilizes modulation techniques such as pulse width modulation (PWM) to control the switching and to synthesize waveforms having desired amplitudes and frequencies.
In order to convert the command signal to the desired output voltage, the motor drive includes a control section. The control section may vary in complexity according to the performance requirements of the motor drive. For instance, a motor drive controlling operation of a pump may only need to start and stop the pump responsive to an on/off command. The motor drive may require minimal control such as an acceleration and deceleration time for the pump. In contrast, another motor drive may control a servo motor moving, for example, one axis of a machining center or an industrial robotic arm. The motor drive may need to not only start and stop the motor, but operate at various operating speeds and/or torques or follow a position command. The motor control may include multiple control loops, such as a position, velocity, torque, or current control loop, an observer, or a combination thereof. Each control loop may include, for example, a proportional (P), integral (I), or derivative (D) controller and an associated controller gain value for each controller in the control loop and may further require additional feedback and/or feed forward controller gain values. In order to achieve the desired operating performance of the motor, it is necessary to properly select the controllers and the associated controller gain values associated with each control loop.
However, selecting the controllers and associated controller gain values may be a complex process. Adjustment of a controller gain value in one control loop may impact performance of another control loop. Although the control loops may be in parallel or in series with each other, there is ultimately a single input and a single output for the control system. Adjusting a controller gain value along one loop impacts the performance of one or more other controller gain values. The interaction of controller gain values often requires a time and labor-intensive iterative approach to selecting gain values in order to achieve the desired level of performance.
As is further known to those skilled in the art, control of the motor is impacted by the load applied to the motor. Motors are connected to mechanical systems to control movement. Every mechanical system generates unique dynamic behavior due to unique mechanical loading on the motor. Mechanical loading is the result of a combination of mechanical system properties that are present in the mechanical system, such as inertia, friction, compliance, backlash, gravity, torque disturbances, machine-to-machine variations, manufacturing tolerances, and slow degradation over time. The effect of this mechanical loading is that the control loop and/or observer gain values need to be calculated as functions of quantities representing each mechanical property. However, these quantities are unknown and skilled technicians must manually adjust the gain values for several control loops and/or an observer to achieve satisfactory performance. This manual tuning process takes time, requires a high skill level, and adds significant cost.
The dynamics of a load connected to the motor typically requires further adjustment of gain values. A load that is rigidly coupled to the motor usually requires different gain values than a load that has a compliant coupling or a coupling with backlash between the load and the motor. Gain values are lowered as the level of compliance and/or backlash increases. As the ratio of load inertia to motor inertia increases, the effects of compliance and backlash are amplified. For example, a coupling with a small level of compliance and a high ratio of load inertia to motor inertia typically results in significantly lower controller gain values than for the same coupling and a low ratio of load inertia to motor inertia. Controller gain values that produce desired performance with a rigidly coupled load may excite resonant operating points with a compliantly coupled load. Consequently, varying levels of compliance and/or backlash result in a unique set of controller gain values for each application. Successfully setting the controller gain values to achieve a desired level of performance for each application typically takes time and requires a skilled technician. Many companies don't have such a technician and may need to hire a field service technician from the manufacturer of the motor drive. This can add significantly to the cost to start up and commission a new control system. Some companies may elect to set the controller gain values to a reduced performance level to ensure stability of the controlled system. However, the reduced performance level may result in lost revenue during operation due to operating at less than maximum capacity.
Presently, there are no methods of determining gain values for the control loops and/or observer that provide initial satisfactory performance over the wide range of mechanical systems found in industry. Also, there are currently no methods of determining gain values that can be applied to mechanical systems where mechanical properties change over time.
Historically, there have been methods developed that claim to provide higher performance of the motor drive. However, these methods typically sacrifice ease-of use. The gain values for the control loops and/or observer are calculated based on quantities for inertia, compliance, backlash, friction, or gravity that are taken from models or assumed to be known in theory. However, these quantities are typically not known by users in practice but rather require considerable effort and technical skill to measure which ultimately resorts to manual tuning procedures.
Other methods have been developed that claim to provide ease-of-use. However, these methods typically sacrifice performance. These methods often claim great initial performance or claim to account for compliance and suppress vibration. However, satisfactory performance is sporadic in practice or the tuning applies to one specific type of machine, which constitutes a very small sub-set of machine types present in industry.
Thus, it would be desirable to provide a motor drive with initial settings for gain values which result in improved tuning-less performance for customers.
Some motor drives may provide an auto-tuning function which attempts to compensate for loading of the motor. Auto-tune methods commonly perform a physical motion test to calculate total inertia and may determine friction as well. The auto tune method then calculates initial gain values for the control loops and/or observer as functions of total inertia, friction, and one bandwidth value. Although these auto-tune methods may provide some success and reduce the need for manually adjusting gain values for the control loops or observer when the motor is connected to a rigid mechanical system, most mechanical systems are not ideal rigid mechanical systems. A rigid mechanical system includes no compliance, backlash, or vibration in the motor, load, or coupling between the motor and load. In contrast, most mechanical systems possess unknown levels of compliance, backlash, and/or vibration, which causes unsatisfactory performance and forces users to manually adjust the gain values for the control loops and/or observer.
Thus, it would also be desirable to provide a motor drive that provides initial settings which provide acceptable performance across a broad range of mechanical systems without requiring further auto and/or manual tuning.